In general, when urea is thermally decomposed at 130° C.-200° C., a condensation product including biuret is formed. Specifically, when the thermal decomposition is performed under reduced pressure, or ammonia that is a product of the thermal decomposition of urea is removed by passing air and nitrogen through a reactor, the thermal decomposition reaction is efficiently carried out so that biuret can be obtained in a great amount in a short period of time. In addition, as the thermal decomposition reaction is performed at high temperature, biuret can be formed quickly but more byproducts, such as a cyanuric acid, triuret, and melamine, may be formed (“Biuret and Related Compounds”, Chemical Reviews, 56, pp. 95-197 (1956).)
A process of thermally decomposing urea to form biuret will now be described in detail.
Urea is heated to 130° C. or higher in a thermal decomposition reactor to form ammonia and a isocyanic acid as illustrated in Reaction Scheme 1:H2NCONH2→NH3+HNCO.  <Reaction Scheme 1>
Reaction Scheme 1 proceeds as urea absorbs heat. As the heating temperature is increased, urea is decomposed more quickly and isocyanic acid is formed more quickly. And as more ammonia is removed, more forward reaction occurs
The isocyanic acid reacts with urea remaining in the reactor to form biuret according to Reaction Scheme 2:HNCO+H2NCONH2→NH2CONHCONH2.  <Reaction Scheme 2>
The reaction of Reaction Scheme 2 is an exothermic reaction. As the temperature is increased, the isocyanic acid which exists in a great amount in an equilibrium state reacts with urea to form biuret. But at this time byproducts, such as a cyanuric acid and triuret are also formed
Such processes for forming biuret have already been disclosed in many references which primarily focus on methods of recovering biuret in large amounts from the byproducts after biuret is formed.
U.S. Pat. No. 3,057,918 discloses a method of recovering biuret in which a crud biuret including a cyanuric acid and triuret is digested with 10% or more aqueous ammonia solution at 80° C.-110° C., and then cooled to obtain a crystal biuret having low solubility with respect to water. The US patent publication also discloses a method recovering a cyanuric acid having high purity by removing ammonia under reduced pressure from a solution from which biuret has been recovered The method described above, however, requires a separate complex device to maintain a concentration of the ammonia in the solution to a predetermined level when the digesting process is performed under pressure. In addition, the digesting process should be performed for a relatively long period of time, such as from 30 minutes to 2 hours. Furthermore, the cyanuric acid should be recovered under reduced pressure. That is, the method described above is expensive.
U.S. Pat. No. 1,324,277 discloses a method of forming biuret having high purity in which urea is dissolved in glycol ether and reacts at 110° C.-210° C. to obtain biuret having high purity. This method, however, includes a process of decomposing urea and a process of reducing the amount of residual urea to a level lower than a desired amount by decreasing the temperature, wherein the process of reducing the amount of residual urea should be performed for 2 to 6 hours. That is, this method also is expensive.
Accordingly, there is a need to develop an inexpensive method in which urea is thermally decomposed using a relatively simple method for a short period of time to obtain biuret having high purity.